A mild synthesis of bis(indolyl)methanes using a deep eutectic solvent

Deep eutectic solvents (such as the combination of urea and choline chloride) are effective solvents/organocatalysts for the condensation of indole and aryl or alkenyl aldehydes to form bis(indolyl)methanes. The reaction conditions are quite mild and do not require additional Bronsted or Lewis acid catalyst, though they fail with ketones or aliphatic aldehydes. Given the inexpensive, non-toxic, and recyclable nature of the DES, these reaction conditions are simple and highly environmentally friendly.     

Flame inhibition by phosphorus-containing compounds in lean and rich propane flames

Chemical inhibition of laminar propane flames by organophosphorus compounds has been studied experimentally and computationally using a detailed chemical kinetic reaction mechanism. Both fuel-lean and fuel-rich propane flames were studied to examine the role of equivalence ratio in flame inhibition. The experiments examined a wide variety of organophosphorus compounds. We report on experimental species flame profiles for tri-methyl phosphate (TMP) and compare them with modeled species flame profile results of TMP and di-methyl methyl phosphonate (DMMP). Both experiments and kinetic modeling indicate that inhibition efficiency is effectively the same for all of the organophosphorus compounds examined, independent of the molecular structure of the initial inhibitor molecule. Chemical inhibition is due to reactions involving small P-bearing species HOPO₂ and HOPO produced by the organophosphorus compounds (OPCs). Ratios of HOPO₂ and HOPO concentrations differ between lean and rich flames, with HOPO₂ dominant in lean flames while HOPO dominates in rich flames. Resulting HOPO₂ and HOPO species profiles do not significantly depend on the initial source of the HOPO₂ and HOPO, and thus are relatively insensitive to the initial OPC inhibitor. A more generalized form of the Twarowski mechanism is developed to account for the results observed, and new theoretical values are determined for heats of formation of the important P-containing species, using the BAC-G2 method.     

An analysis of the reflective component of small-angle X-ray scattering patterns from crazed polystyrene

An investigation of the reflective component of small-angle x-ray scattering patterns of polystyrene crazes is performed. It is shown that the strong streak parallel to the tensile axis consists predominantly of reflected radiation, while the remaining pattern is composed entirely of diffracted radiation. X-ray reflection off unparallel regions of crazes is discussed and the nature of the reflection is also considered in terms of collimation quality. An analysis methodology based on craze tip angular distribution is proposed.     

Acetaldehyde oxidation in the negative temperature coefficient regime: Experimental and modeling results

Acetaldehyde oxidation has been studied in experiments at temperatures of 553 and 713 K carried out in a low pressure, static reactor and in numerical modeling calculations using a detailed chemical kinetic reaction mechanism. The results of the experimental study were used to construct and validate the reaction mechanism, which was then used to examine acetaldehydeoxidation in the negative temperature coefficient regime between 550 and 900 K. This mechanism was also tested against independent measurements of acetaldehyde oxidation carried out by Baldwin, Matchan, and Walker. The overall rate of reaction and the properties of the negative temperature coefficient regime were found to be sensitive to the competition between radical decomposition reactions and the addition of molecular oxygen to acetyl and methyl radicals, including particularly During these experiments, an upper limit to the rate of decomposition ofCH₃O₂H was measured at 553 K. Implications of the results for future kinetic modeling of engine knock are discussed.     

Towards a monolithic optical cavity for atom detection and manipulation

We study a Fabry-Perot cavity formed from a ridge waveguide on a AlGaAs substrate. We experimentally determined the propagation losses in the waveguide at 780 nm, the wavelength of Rb atoms. We have also made a numerical and analytical estimate of the losses induced by the presence of the gap which would allow the interaction of cold atoms with the cavity field. We found that the intrinsic finesse of the gapped cavity can be on the order of F∼30, which, when one takes into account the losses due to mirror transmission, corresponds to a cooperativity parameter for our system C∼1.     

Fiber-based tunable dispersion compensation

Tunable dispersion has been implemented in various technology platforms, including fiber gratings, planar waveguides, thin film etalons, and bulk optic technologies. This paper will focus on fiber grating based tunable dispersion compensation, because fiber gratings are at present one of the best developed TDC technologies available. The paper is divided into three parts. In the first part we describe grating based TDC technologies and discuss their advantages and disadvantages. We focus on thermally tuned linearly chirped fiber gratings, as these have to date been the most successful grating technology for 40 Gbit/s. We also compare grating TDCs to two other prominent tunable dispersion technologies: thin film etalons and planar waveguide ring resonators. In the second section we describe the techniques used to fabricate high performance dispersion compensation gratings as well as the theory of the primary defect of fiber grating dispersion compensation: group delay ripple (GDR). In the third section we describe the telecom system related issues for tunable gratings, including characterization of grating performance, tunability requirements and results from actual system trials using tunable FBGs.     

Realizing a stable magnetic double-well potential on an atom chip

We discuss design considerations and the realization of a magnetic double-well potential on an atom chip using current-carrying wires. Stability requirements for the trapping potential lead to a typical size of order microns for such a device. We also present experiments using the device to manipulate cold, trapped atoms.     

Experimental evidence for the breakdown of a Hartree-Fock approach in a weakly interacting Bose gas

We investigate the physics underlying the presence of a quasicondensate in a nearly one dimensional, weakly interacting trapped atomic Bose gas. We show that a Hartree-Fock (mean-field) approach fails to predict the existence of the quasicondensate in the center of the cloud: the quasicondensate is generated by interaction-induced correlations between atoms and not by a saturation of the excited states. Numerical calculations based on Bogoliubov theory give an estimate of the crossover density in agreement with experimental results.     

Magnetic trapping of metastable helium atoms

Metastable helium atoms were efficiently transferred from a magneto-optical trap (MOT) to a magnetic quadrupole trap, producing samples of up to 3᎒⁷ magnetically trapped atoms at a temperature of about 1 mK. We observe purely exponential decay of the samples with time constants of 9-10 s and derive an upper bound for the rate coefficient of inelastic Penning collisions.     

Two body loss rate in a magneto-optical trap of metastable He

We have measured the two body loss rate in a magneto-optical trap containing triplet metastable He atoms. We find a rate constant cm³/s at a -8 MHz detuning, with an uncertainty of a factor 2. This measurement is in disagreement with a recent experiment which measures the absolute, ion-producing collision rate, but agrees with several other published measurements. Received 20 April 1999 and Received in final form 12 July 1999